Byrne's run on FF brought us Sue Richards with
a mullet. Um ... seemed like a good idea at the time?
I've said it before, but having recently read most of Byrne's FANTASTIC FOUR run, I think the first half was really nicely drawn, with the quality starting to decline during the second half.
If I had to guess, I'd say Byrne was overextending himself toward the end, writing and drawing FANTASTIC FOUR and ALPHA FLIGHT and writing THE THING and doing whatever else.